1. Field of the Invention
The present invention generally relates to methods of setting communication bands and communication devices, and, more particularly, to a method of setting communication bands in the ascending direction (transmitting direction) and the descending direction (receiving direction) and a communication device that operates by that method.
2. Description of the Related Art
In recent years, more and more people have been accessing the Internet not only from offices and schools but also from general households. To make an access to the Internet from a general household, a user has to use a telephone service as a communication line. However, since a telephone service is generally aimed at audio signal transmission, the upper limit of the frequency bandwidth is 4 kHz, and the data transmission speed is restricted accordingly.
To eliminate this problem, an xDSL (x Digital Subscriber Line) system has been employed for communication on the Internet. In the xDSL system, signals having frequencies higher than 4 kHz are transmitted through a telephone line so as to achieve high-speed data communication using neither transmission apparatus nor switching apparatus of a telephone network.
FIG. 1 is a block diagram of an xDSL modem that utilizes a DMT (Discrete Multi-Tone) modulation technique.
An xDSL modem 10 shown in FIG. 1 comprises a data divider 12, a data synthesizer 14, QAM (Quadrature Amplitude Modulation) modulators 16-1 through 16-m, QAM demodulators 18m+1 through 18n, a synthesizer 20, a divider 22, a multiplexer 24, and a D-A/A-D converter 26.
The data divider 12 divides inputted transmission data, taking the influence from noise on the line into consideration, and supplies the divided transmission data to the QAM modulators 16-1 through 16-m. 
The QAM modulators 16-1 through 16-m modulate the supplied transmission data to generate sub carriers 1 through m each having a bandwidth of about 4 kHz. The synthesizer 20 compounds the sub carriers 1 through m, and transmits the compound to the public switched telephone network via the multiplexer 24 and the D-A/A-D converter 26.
The divider 22 divides a reception signal inputted from the public switched telephone network via the D-A/A-D telephone network 26 and the multiplexer 24 so as to generate sub carriers m+1 through n. The QAM demodulators 18-m+1 through 18-n demodulate the sub carriers m+1 through n, and supply the demodulated data to the data synthesizer 14. The data synthesizer 14 compounds reception data from the demodulated data, and outputs the reception data.
FIG. 2 shows the structure of a frequency spectrum in a DMT modulation system. In FIG. 2, a frequency band fa to fb is fixed in the ascending direction (transmitting direction), and a frequency band fc to fd is fixed in the descending direction (receiving direction). For instance, an ADSL (Asymmetric Digital Subscriber Line) system, which is one of xDSL systems, has a descending frequency band wider than its ascending frequency band, in order to be compatible with an access to the Internet from a general household.
In the xDSL modem 10, however, the ascending frequency band and the descending frequency band cannot be changed in accordance with the relationship between the amount of transmission data and the amount of reception data, because both the ascending frequency band fa to fb and the descending frequency band fc to fd are fixed as shown in FIG. 2.
When a user accesses the Internet from a general household, a wide descending frequency band is required for receiving a large amount of data from a homepage, while only a narrow ascending frequency band is used for transmitting request data on the Internet. Still, the conventional xDSL modem 10 cannot change the allocation of the frequency bands in the ascending and descending directions, resulting in poor usage efficiency of the ascending frequency band.